1. Field of the Invention
The present invention relates to a rolling bearing for supporting a spindle shaft of a machine tool that rotates at a high speed, and a spindle apparatus for machine tool using the rolling bearing.
2. Description of the Related Art
The conventional art will be described with a cylindrical roller bearing as an example. A bearing for supporting a spindle shaft of a machine tool requires high rotational accuracy and rigidity in order to keep the machining accuracy high. The cylindrical roller bearing used for this purpose is usually used with an inner clearance of the bearing adjusted to a small clearance of about 5 xcexcm after the roller bearing was incorporated into the spindle shaft and a housing.
On the other hand, there is a recent tendency to use the spindle apparatus at a high speed in order to improve the machining efficiency, and it is necessary for the bearing to correspond to the speed-up. As a bearing capable of corresponding to this request, a cylindrical roller bearing having an inner ring with ribs and an outer ring without ribs (N type) is used more widely, which can more readily exhaust the supplied lubricating oil to the outside of the bearing than a cylindrical roller bearing having an outer ring with ribs and an inner ring without ribs (NU type). Further, as a retainer guide type (retainer positioning type) that is widely used at the present time, there are an inner ring guide type in which the retainer is made of a copper alloy and guided by the outer surface of the ribs of the inner ring, and a roller guide type in which the retainer is made of synthetic resin and guided by the rollers.
Further, in the conventional rolling bearing, rolling elements such as balls, rollers, or the like are interposed between raceway surfaces formed on the outer surface of the inner ring and the inner surface of the outer ring. In this rolling bearing, in order to retain the rolling elements on the raceway surface at equal intervals in the bearing circumferential direction, an annular retainer is interposed between the outer ring the inner ring.
A bearing for spindle apparatus of the machine tool that is an example of the rolling bearing requires low vibration and low noise in order to improve machining accuracy. Further, the bearing recently requires high-speed rotary performance (that it can be stably used at a high rotational speed for a long time) with grease lubrication that is effective from viewpoints of easiness of handling, environment and cost. In order to satisfy this characteristic, conventionally, there has been used a retainer which is light, superior in softness, and made of synthetic resin to which reinforced fibers are added for improvement of the strength.
FIGS. 34 and 35 shows a conventional roller bearing using a retainer of an outer ring guide type and both-side guide. FIG. 35 is a sectional view taken along a line of XXXIVxe2x80x94XXXIV in FIG. 34. A roller bearing 200 is schematically composed of an outer ring 201, an inner ring 202, rolling elements 203, and a retainer 204.
As shown in FIG. 35, the axial both ends of the inner ring 202 has ribs 202a that protrude toward the outer ring side in the radial direction.
The retainer 204 is made of synthetic resin, and has pocket portions 205 for retaining the rolling elements 203 in its circumferential direction at intervals. Further, the retainer 204 has sidewalls 204b, 204b that are erectly provided on the axial both sides toward the outer ring side in the radial direction. The leading end surface of the sidewall 204b, which is opposed to the outer ring 201, i.e. a guided surface 204a in this example, comes into sliding contact with the outer ring 201 or comes into close contact with it. Since the retainer 204 is guided (slide-guided) by the slide-contact of the guided surface 204a with the inner surface (guide surface) of the outer ring 201, the retainer 204 rotates relatively with respect to the outer ring 201.
Further, the retainer made of synthetic resin described the above is formed by injection-molding, and has been applied to a bearing for machine tool that frequently runs with the very small amount of grease or lubricant oil to be used in order to minimize heat-generation at the operating time. Hereby, in the bearing for machine tool, the essentially minimum amount of lubricant is used, to thereby suppress agitation resistance of softener such as the grease or the lubricant oil, and heat generation due to the agitation resistance. As a method for manufacturing such the synthetic resin-made retainer, there is a radial draw type of an injection-molding method.
FIG. 36 shows a partially sectional view of a forming mold 210 used in the radial draw type of injection molding method. A part of the forming mold 210 is schematically composed of a movable mold 211, a first slide core 212, and a second slide core 213.
The movable mold 211 has an outer surface with a cylindrical shape that forms an inner surface of the retainer. At the outer periphery of the movable mold 211, plural slide cores (not shown) are arranged, which form an outer surface of the retainer and pocket portions for retaining rolling elements at the mold tightening time. The first slide core 212 and the second slide core 213 of the plural slide cores are formed in a convex shape in section, and they are composed of circular-arc shaped base portions 212a, 213a located along with the outer surface of the movable mold 211 and columnar protrusions 212b, 213b that are erectly provided substantially perpendicularly toward the surfaces on the movable mold 211 side of the base portions and form the pocket portions of the retainer.
When the forming mold 210 is tightened, the protrusions 212b, 213b of the slide cores 212, 213 come into contact with the predetermined positions on the outer surface of the movable mold 211, and the adjacent side surfaces 212c, 213c of the base portions 212a, 213a of the slide cores comes into contact with each other, whereby a cavity space of the retainer is formed. Then, by injecting the synthetic resin in the cavity space, an annular retainer 220 made of the synthetic resin is formed, which has the pocket portions arranged in the circumferential direction at intervals.
FIG. 37 shows a side view of the retainer 220 formed by the forming mold 210. A parting line PL, which has been formed by bringing the side surfaces 212c, 213c of the slide cores 212, 213 into contact with each other at the injection-molding time, is formed between the adjacent pocket portions of the plural pocket portions 221 to each other on the outer surface of the retainer 220. FIG. 38 is a plan view of the retainer 220 viewed from a direction XXXVII in FIG. 37.
Moreover, in the rolling bearing in which the guide type of the retainer is an inner ring guide type or an outer ring guide type, a appropriate clearance (guide clearance) is provided between the inner diameter surface of the retainer (or outer diameter surface) that comes into contact with a raceway ring (the inner ring and outer ring) and a contact surface of the raceway ring. The guide clearance is set to be a difference between a diameter of a retainer guide surface of the raceway ring and a diameter of the guided surface of the retainer. Further, in order to guide and retain the rolling element, a clearance (pocket clearance) is provided between the rolling element in the pocket of the retainer and the pocket.
Conventionally, in this type of rolling bearing used for high-speed rotation, a guide clearance 5 of the retainer 4 is 0.4 to 1.0%of a guide diameter d1 (a retainer inner diameter 4a), and a pocket clearance 6 is 2 to 4% of the ball diameter, which is 0.5 to 2 times the above guide clearance 5 (FIG. 39).
In case that the bearing rotates at a high speed at the operating time of the machine tool, grease filled inside the bearing or lubricating oil supplied by oil-air lubrication is scattered to the outer ring side by centrifugal force, so that in case of the inner ring guide retainer, there is a possibility that lubricant defect of the guided surface causes seizure. Further, at the time of the high-speed rotation, due to expansion of the inner ring groove and outer cylinder cooling by the centrifugal force, and the increase of the inner/outer ring temperature difference by the influence of a motor built in a machine, the inner clearance of the bearing becomes negative during running, so that there is also a problem that roller load increases. Further, the retainer is whirled by the centrifugal force at the time of the high-speed rotation, and the retainer made of the synthetic resin has also a problem that the amount of deformation is large in case that the strength of the retainer is low. Particularly, in case of the roller guide retainer, since the retainer holds the rollers therein and is guided by the rollers, when whirling of the retainer is produced by the influence of the centrifugal force or when the amount of deformation is large, the roller is restrained in the pocket portion, and its operation is obstructed. Further, the excessive load is applied onto the retainer by the rollers, so that there is fear that the retainer is broken.
In case that the bearing 30 having the above outer ring guide retainer is used as the bearing for spindle apparatus of the machine tool, when the spindle shaft is run at a high speed, grease filled in the bearing is scattered to the outer ring side of the bearing by centrifugal force, so that the scattered grease is raked out to the side of the retainer by the retainer and comes to form a bank. By leakage of a very small amount of base oil from this state, the rolling surface and the guided surface of the retainer are lubricated. However, since the guided surface 34a of the retainer 34 is slide-guided by the inner surface of the outer ring 31 (or the outer surface of the inner ring 32), when an oil film runs short, the bearing causes wear of the guided surface 34a of the retainer 34, and wear of the outer ring 31 or the inner ring 32 by the reinforced fiber added in order to improve the strength of the retainer 34. At this time, there was a possibility that wear powders produced in the raceway surface and the guided surface 34a deteriorate the grease.
Further, recently, with speed-up of the spindle rotation of the machine tool, it is not rare that the spindle shaft is rotated at a region in which, for example, a Dmn value (bearing pitch circle diameter (mm)xc3x97rotation speed (rpm)) is over 1xc3x97106. In case of such the high-speed rotation, since the grease is raked out to the outside of the guided surface by the centrifugal force, the grease cannot be held on the guided surface 34a of the retainer 34. Therefore, it is difficult to form the oil film on the guided surface 34a of the retainer 34. And, not only the guided surface 34a is worn but also the inner surface of the outer ring 31 opposed to the retainer 34 is worn by the reinforced fiber added to the retainer 34 for improvement of the strength, so that the grease is early deteriorated by the wear powders produced at this time.
In case of the inner ring guide type of the retainer, the surface of the sidewall of the retainer, opposed to the inner ring, becomes a guided surface, and the outer surface of the inner ring, opposed to the guided surface becomes a guide surface. In this case, when the inner ring rotates, the grease on the slide-guided surface of the retainer is scattered to the outer ring side by the centrifugal force. Therefore, lubricating property between the guided surface of the retainer and the guide surface of the inner ring remarkably lowers, which causes early breakage of the retainer and the inner ring.
Further, as shown in FIG. 38, the parting line PL formed on the outer surface of the retainer 220 by the radial draw type of the injection-molding method has a burr projecting in the radial direction of the retainer and protruded burr extending axially. This burr, while the retainer 220 is incorporated into the bearing and the bearing is operating, comes into slide-contact with the inner surface of the outer ring of the bearing and wears thereby to produce wear powders, or causes a torque variation thereby to disturb the rotation of the bearing.
Conventionally, the burr was removed by cutting or barrel machining before the injection-molded retainer was incorporated into the bearing by removing operation, whereby the rotation of the bearing was made good.
However, the burr removal working is complicated, which causes the increase of cost of the rolling bearing.
Further, regarding the clearance of the retainer, since the conventional guide clearance 5 is large so that it is 0.4 to 1.0% of the guide diameter d1, when the retainer 4 whirls at the time of the high-speed rotation, the retainer 4 causes balls 3 to be arranged unequally, which gives an influence onto run-out accuracy of a shaft.
Frequency of this run-out (hereinafter, this can be referred to as xe2x80x9cFc run-outxe2x80x9d) is a rotational frequency of the retainer, and different from a rotational frequency of the shaft.
In case that the bearing is used in the spindle apparatus for machine tool, if this FC run-out is produced, there is a possibility that an influence may be given on a problem of worsening the external appearance of the machine surface (problem of generating a sink mark).
Specifically, FIG. 39 shows a schematic diagram of a general rolling bearing of the inner ring guide type. The retainer 4 is rotationally moved while being guided by the retainer guide surface 2a (outer diameter d2) of the inner ring 2. Particularly in case of high-speed rotation, with unbalance of the retainer 4 as a start, the retainer 4 starts this rotational movement.
By this rotational movement, since a center of the pocket 4b is out of the equal layout position of the ball 3, the unequal layout of the ball 3 is produced. In the figure, reference numeral 1 represents the outer ring, reference character P represents an equal layout position of the ball, and reference character R represents an unequal layout angle of the ball.
Further, as shown in FIG. 39, in case that the pocket clearance 6 is smaller than the guide clearance 5, even if the ball 3 intends to return to the equal layout position, it cannot return there, so that the unequal layout becomes large.
By this unequal layout of the ball 3, the radial rigidity of the bearing changes in the circumferential direction, and the displacement is produced in the center of the shaft. This displacement is produced with the same frequency as the rotational frequency of the retainer, and causes the Fc run-out.
The invention has been made in view of these problems, and an object of the invention is to provide a rolling bearing in which the lubrication property is improved, and deformation and breakage of the retainer can be suppressed, and to a spindle apparatus for machine tool using it.
Further, the object of the invention is to provide a rolling bearing in which the lubrication property can be improved and high-speed stability can be secured.
In addition, the object of the invention is to provide a rolling bearing in which working steps can be reduced and operation defect is not produced.
Moreover, the object of the invention is to reduce generation of the Fc run-out of the shaft produced by whirling of the retainer at the time of the high-speed rotation.
In order to achieve such the object, there is provided a rolling bearing, comprising:
an inner ring having an outer surface;
an outer ring having an inner surface;
a plurality of rolling elements rotatably disposed between the inner ring and the outer ring; and,
a retainer for retaining the rolling elements, the retainer being made of a resin material, the retainer being positioned with respect to the inner surface of the outer ring or the outer surface of the inner ring, the retainer including a pair of annular portions axially arranged in parallel and a columnar portion coupling the annular portions,
wherein the following expression is obtained:
AI=LH3/dm2xe2x89xa70.025xe2x80x83xe2x80x83(1) 
in which H is a radial length of a section of the annular portion, L is an axial length of the same, and dm is PCD of the rolling element.
According to this invention, the retainer is positioned with respect to the inner surface of the outer ring, that is, the guided surface of the retainer is provided in the retainer outer surface opposed to a raceway surface of the outer ring. Therefore, a clearance between the outer surface of the inner ring and the inner surface of the retainer, or a clearance between the inner surface of the outer ring and the outer surface of the retainer can be designed to be comparatively large. As a result, lubricating oil ejected from a nozzle for oil-air or oil-mist lubrication can be surely introduced into the bearing from the comparatively large clearance, and the lubricating oil is scattered to the outer ring side by centrifugal force, so that the guided surface of the retainer is lubricated smoothly.
The retainer is guided thus by the outer ring or the inner ring, whereby the whirling amount of the retainer at the time of the high-speed rotation can be controlled by the guide clearance. Further, as a material of the retainer, there is used a resin material in which the seizure is hardly caused even with a slight amount of lubrication, and the above expression (1) is satisfied, in which H is the radial length of the section of the annular portion, L is the axial length of the same, and dm is PCD of the rolling element, whereby higher rigidification becomes possible than in the conventional retainer, and the amount of deformation at the high-speed rotating time can be suppressed.
As the resin material of the retainer, phenol resin, polyamide 46, polyamide 66, polyphenylene sulfaid, thermoplastic polyimide, polyether etherketone, and the like can be used as base material. Further, it is preferable to add glass fiber by 10-40 wt %, carbon fiber by 10-30 wt %, or aramid fiber by 10-30 wt % in order to improve strength of the retainer. Further, in order to satisfy use in high-speed rotation, the carbon fiber or the aramid fiber is more preferable. However, according to the use, the glass fiber can be also selected. In case that the addition amount of the carbon fiber or the aramid fiber is 10 wt % or less, the strength cannot be sufficiently maintained. In case that it is 30 wt % or more, the molding property is deteriorated and the external appearance is also bad. Further, more preferably, the addition amount of the carbon fiber or the aramid fiber is 20-30 wt %, whereby both the strength and the molding property become improved. The addition amount of the glass fiber is preferably 10-40 wt %, and this reason is the same as the above reason.
However, in case of the cylindrical roller bearing, if the axial width of the retainer of the outer ring guide type is set so as to satisfy the above expression (1), there is a fear that a lead-in portion of the outer ring raceway surface, which is provided in order to smoothly perform the incorporation of the cylindrical rollers, interferes with the guided surface of the retainer by the axial movement of the inner ring due to an incorporation error and shaft expansion during running. Hereby, such a disadvantage is caused that the brake is applied on running of the retainer and the guided surface of the retainer wears. Against this, chamfering and tapering are applied to the peripheral portion of the guided surface of the retainer, and a clearance of 0.5 mm or more is provided axially up to an intersecting point of the raceway surface of the outer ring and the lead-in portion, whereby this disadvantage can be avoided.
Further, a notch is provided for at least one of four corners in each pocket of the retainer for retaining the rolling elements, whereby it is possible to speedily move the lubricating oil supplied to the raceway surface of the inner ring and the ribs, according to the rotational speed of the bearing, through the notch to the raceway surface of the outer ring.
Further, a roller or ball guide portion in circumferential direction of the pocket has a flat surface, which is parallel to the shaft which the bearing supports. Thus, a snap portion for retaining the roller or the ball, in view of the moving amount of the retainer and dimensional tolerance, can be formed so that it does not interfere with the roller or the ball at worst during running, and the high-speed rotation can be realized without obstructing the movement of the roller or the ball. In addition, in order to obtain high-speed stabilization of the retainer, the both-side guide effective for inclination of the retainer is preferable.
Further, in order to attain the above object, there is provided a rolling bearing comprising:
an inner ring having an outer surface;
an outer ring having an inner surface;
a plurality of rolling elements rotatably disposed between the inner ring and the outer ring; and,
a retainer for retaining the rolling elements, the retainer being made of a resin material, the retainer including a guided surface that is guided by the inner surface of the outer ring or the outer surface of the inner ring,
wherein the guided surface has two or more recesses.
The guided surface of the retainer is opposed to either the inner surface of the outer ring or the outer surface of the inner ring, and located on circumference of the retainer that comes into slide-contact with the outer ring or the inner ring.
According to this rolling bearing, even when it is used in such a high-speed rotation region that a Dmn value is over 1xc3x97106, not only the grease can be held on the guided surface of the retainer made of synthetic resin but also an oil film by the grease is appropriately formed since a wedge effect is produced in the recess portion. Namely, the clearance between the guided surface of the retainer and the guide surface of the raceway ring is large in the point where the recess is provided and small in other points than it, and the grease held in the recess is pulled into the points where the clearance is small. As a result, it is prevented that the guided surface of the retainer is worn by the slide with the raceway surfaces of the outer ring and inner ring, and the rolling bearing that is superior in high-speed stability (low-torque, low-noise, low-vibration, seizure-resistance) can be provided. Further, since the rolling bearing of the invention can suppress the occurrence of the iron powder due to wear, the deterioration of the grease is prevented, so that a long use becomes possible.
In the above constitution, a depth of the recess portion in the radial direction of the retainer is preferably 0.3 mm or more. Hereby, on the guided surface of the retainer made of synthetic resin, the enough grease can be held.
Moreover, to attain the above object, there is provided a rolling bearing, comprising:
an inner ring;
an outer ring;
a plurality of rolling elements rotatably disposed between the inner ring and the outer ring; and
a retainer for retaining the rolling elements, the retainer being made of a resin material and formed by an injection-molding, the retainer having a recess axially extending on an outer periphery (an outermost surface) thereof,
wherein the recess has a parting line formed at the time of injection-molding.
Here, xe2x80x9coutermost surface of the retainerxe2x80x9d means an outer surface located outermost in the radial direction of the annular retainer, that is, an outer surface located outside in the radial direction of the retainer from a parting line formed on the outer surface except for at least the outermost surface.
Further, it is preferable that the recess formed on the outermost surface has such a depth that the parting line formed in the recess portion does not protrude outward of the retainer from the outermost surface.
According to the rolling bearing of the invention, since the parting line formed on the outer surface of the retainer does not protrude from the outermost surface of the retainer, the retainer can be incorporated into the bearing without having necessity to perform after-treatment after injection-molding of the retainer. Accordingly, the assembly process of the rolling bearing can be simplified, and yield of parts of the retainer can be also improved. Further, when the retainer is incorporated into the bearing, the parting line formed on the outer surface of the retainer does not come into slide-contact with the inner surface of the outer ring of the bearing. Namely, during running of the rolling bearing, the bad operation due to wear of the parting line and the torque change is not caused.
Further, in this case, a forming mold comprises a fixed mold for forming one end of the retainer, a columnar movable mold for forming the inner surface of the retainer and the other end, and a plurality of slide-cores that are arranged outside of the movable mold, formed convexly in a section, and form the outer surface of the retainer and the pocket portions. The slide cores have a circular-arc shaped base portion put side by side with the outer surface of the movable mold and a protrusion erectly provided on the surface on the movable mold side of the base portion nearly perpendicularly.
When a forming mold is tightened, the leading end surface of the protrusion of the slide core comes into contact with the outer surface of the movable mold, and side surfaces of the base portions of the slide cores adjacent to each other comes contact with each other, whereby a cavity space is formed at the periphery of the movable mold, and synthetic resin material is injected in the cavity space, so as to form a retainer. In a apparatus of manufacturing a rolling bearing retainer that forms thus the retainer, it is preferable that a projection member that protrudes to the movable mold side is provided on the inner surface of the contact portion of the base portion that becomes an outermost surface of the retainer when the side surfaces of the base portions adjacent to each other come into contact with each other, and that the parting line formed on the outermost surface of the retainer when the synthetic resin is injected is formed in the recess portion formed by the projection member.
According to this apparatus of manufacturing the rolling bearing retainer, since the parting line formed on the outer surface of the retainer does not protrude from the outermost surface of the retainer, it is necessary to perform after-treatment after injection-molding of the retainer. Accordingly, the assembly process of the rolling bearing can be simplified, and yield of parts of the retainer can be also improved.
In addition, there is provided a rolling bearing, including:
an inner ring having an outer surface;
an outer ring having an inner surface;
a plurality of rolling elements rotatably disposed between the inner ring and the outer ring; and
a retainer for retaining the rolling elements, the retainer being made of a resin material, the retainer including a guided surface that is guided by the inner surface of the outer ring or the outer surface of the inner ring and a pocket holding the rolling element,
wherein a guide clearance between the inner surface of the outer ring or the outer surface of the inner ring and the guided surface of the retainer is set to 0.05-0.4% of a diameter of the guided surface of the retainer, and
wherein a clearance between the pocket of the retainer-and the rolling element is set to 0.8-1.8% of the guide clearance.